The invention relates to a process for producing grain-oriented magnetic steel sheeting in which a slab made from a steel containing (in mass %) more than 0.005 to 0.10% C, 2.5 to 4.5% Si, 0.03 to 0.15% Mn, more than 0.01 to 0.05% S, 0.01 to 0.035% Al, 0.0045 to 0.012% N. 0.02 to 0.3% Cu, the remainder being Fe, including unavoidable impurities, is heated through at a temperature below the solubility temperature for manganese sulphides, at any rate however below 1320.degree. C. but above the solubility temperature for copper sulphides; subsequently hot rolled to a final thickness of the hot strip between 1.5 and 7.0 mm, with an initial temperature of at least 960.degree. C. and with a final temperature in the range of 880 to 1000.degree. C. The hot strip is subsequently annealed for 100 to 600 s at a temperature ranging from 880 to 1150.degree. C. and immediately cooled at a cooling rate in excess of 15 K/s and cold rolled in one or several cold-rolling steps to the final thickness of the cold strip. Subsequently the cold strip is subjected to a recrystallising annealing process in a humid atmosphere containing hydrogen and nitrogen, with synchronous decarburisation, and after application on both sides of a parting agent essentially containing MgO it is annealed at high temperature and after application of an insulating layer it is subjected to final annealing.
Such a process has been disclosed in DE 43 11 151 C1. A reduction in the preheat temperature of the slab to below the solubility temperature of MnS, at any rate however below 1320.degree. C. is possible by using copper sulphide as the significant grain growth inhibitor. Its solubility temperature is so low that even with preheating at this reduced temperature and the subsequent hot-rolling in conjunction with annealing the hot-rolled strip, an adequate formation of this inhibitor phase is possible. Due to its very much higher solubility temperature, MnS does not play a role as an inhibitor, and AlN--whose solubility and elimination properties are in between those of Mn sulphide and Cu sulphide--participates only insignificantly in the inhibition.
The purpose of reducing the temperature prior to hot rolling is to avoid liquid slag deposits on the slabs, thus reducing wear and tear of the annealing plant and increasing production yield.
EP-B-0219 611 describes a process which also allows a reduction in the slab preheating temperature in an advantageous way. In this, (Al, Si) N-particles are used as grain growth inhibitors which are introduced by way of a nitration process to the strip which has been cold-rolled to finished thickness and decarburised. As a measure for carrying out this nitration process, the annealing atmosphere during coarse grain annealing is selected in such a way that it has a nitration ability, or else nitrating additives are used for annealing separation, or a combination of both, is disclosed.
EP-B-0 321 695 describes a similar process. Exclusively (Al, Si) N-particles are used as grain growth inhibitors. Additional details regarding the chemical composition are disclosed and a further possibility of a nitration treatment in conjunction with the decarburisation annealing is shown. Furthermore, it is indicated that the slab preheat temperatures should preferably be kept below 1200.degree. C.
EP-B-0 339 474 also describes a process whereby however nitration treatment in the form of continuous annealing in the temperature range of 500 to 900.degree. C. in the presence of an adequate quantity of NH.sub.3 in the annealing gas is carried out in detail. Furthermore, there is a detailed description as to how the annealing nitration treatment can directly follow the decarburisation annealing. Here too, the aim is to form (Al, Si) N-particles as effective grain growth inhibitors. In this it is emphasised in particular that for such a nitration treatment, at least 100 ppm, preferably however more than 180 ppm of nitrogen must be charged. The slab pre-heat temperature should be below 1200.degree. C.
EP-B-0 390 140 particularly emphasises the special significance of the grain size distribution of the decarburised cold strip and provides various methods for their determination. In each case, a slab preheating temperature of less than 1280.degree. C. is stated. However, there is always the recommendation to preheat the slabs to below 1200.degree. C.; all examples of the process indicate 1150.degree. C. as the preheat temperature.
In comparison, the process known from DE 43 11 151 C1 has the significant advantage that the preheating temperatures do not have to be selected as low as the above-mentioned 1150 to 1200.degree. C. With the often used mixed rolling operation of a modern hot rolling plant, slab preheating temperatures of between 1250 and 1300.degree. C. are often set, because from the point of view of power engineering and hot-rolling technology, this temperature range is particularly favourable. In addition, the use of copper sulphide as an inhibitor has the decisive advantage that one does not have to carry out and master a nitration treatment by an additional technology, but can directly generate the grain growth inhibitor already at the beginning of the production process. In this way, further processing of the hot strip through to the finished product is significantly simplified.